The present invention relates to a method of forming a multi-color image, wherein color separation images are generated in first, second, and possibly subsequent colors, which color separation images are formed by the use of colored toner powder. The color separation images are transferred, in successive image-transfer steps under the influence of pressure, to an elastically deformable adhesive intermediate medium provided with a top layer, and are collected thereon to form a multi-color image. The multi-color powder image is then softened and transferred under the influence of pressure to a receiving support, wherein between two consecutive image transfer steps the powder image consisting of one or more color separation images is so deformed on the intermediate medium, under the influence of heat and/or pressure, that the powder image becomes adhesive for toner powder, so that a subsequent color separation image can be transferred not only to the intermediate medium but also to the powder image already present. The present invention also relates to an apparatus suitable for performing the method according to the invention and to an intermediate belt usable in this apparatus for performing the method according to the invention. In this way it is possible to deposit transparent layers of toner powder in different colors on one another on the intermediate medium and obtain the required color shade via subtractive color mixing.
An apparatus suitable for performing this method is known from NL-A-92 00713.
This apparatus comprises four or more image recording elements, means for generating color separation images consisting of colored toner powder on the image recording elements, an intermediate medium provided with a surface covering of elastically deformable material for collecting the various color separation images thereon to form a multi-color image, and means for pressing against the intermediate medium in a transfer zone at least each image recording element on which the second and each following color separation image is formed.
In practice it has been found that there is in this known apparatus only a very small intermediate belt temperature gradient within which the apparatus operates reliably. If the temperature of the intermediate belt is too low, the toner powder is insufficiently softened, so that a subsequent powder image is not transferred sufficiently to the existing powder image. If the intermediate belt temperature is too high, the toner image from a first image recording element is deposited on a subsequent image support.
It has also been found that the image support gradually rises in temperature during operation of the apparatus so that the apparatus will operate unreliably during this time. The object of the present invention is to drastically reduce the above disadvantages. To this end, according to the present invention, the intermediate medium contains a heat-insulating layer in which the product of the thermal conductivity coefficient .lambda. in J/m sK, the density .rho. in kg/m.sup.3 and the specific heat C.sub.p in J/kg K has a value of less than 2.times.10.sup.5, and preferably less than 1.5.times.10.sup.5. Advantageously, the product of the thermal conductivity coefficient, density and specific heat falls within the range of about 0.1.times.10.sup.5 to less than 2.times.10.sup.5.
As a result, a relatively low contact temperature is obtained between the image supports and the intermediate belt, and this enables toner images to be adhesively collected on a relatively hot intermediate belt without the image support becoming too hot, thus obviating toner particles from adhering firmly to the image support. The heat flow to the image supports is also reduced as a result, so that there is an energy saving. The reduced heat capacity with respect to the heat capacity of the intermediate belts also results in faster heating up of the intermediate belt during the heating operation. Consequently, the apparatus is ready to operate more quickly after starting.
Preferably, the heat-insulating layer is applied directly beneath the top layer in the form of an intermediate layer between the top layer and the base layer. This results in a small temperature gradient over the intermediate layer so that run/standby differences will occur to a reduced degree. Preferably, gas-filled fillers are used in the insulating layer. As a result the thermal conductivity in the belt is reduced to about 0.05-0.15 W/mK. In one embodiment, gas-filled glass beads are provided with a primer, thus giving better adhesion between the glass and the rubber. In order to further reduce the heat flow to the image support, a very short nip time is chosen in the image transfer step between the image support and the intermediate belt. This short nip time can be obtained by means of high speed during the image transfer and, as will be clear to the skilled man, by correct choice of the diameters of the image support and the intermediate belt in the image transfer zone.